I’m at 120k miles in my 2017 S. No AC Problems but just popped a fuse and lost the heater core. Considering the amount of labor to replace the fuse, and the fact that the core has been through several revisions since my car was new, better to just have them upgrade the core to the newest revision when they replace the fuse.A/C compressor replacement on our Model S was over $3k last year, mostly labor, so unfortunately $4k for the more complicated heat pump sounds about right. The Model S A/C lasted well over 100k miles for us though.
We did have a problem with its resistive heater while it was still under 100k mile warranty, but I forget what the resolution was, I don't even remember if the heater itself needed replacing or if it was something simpler. Let's just say as an early Model S it's seen a lot of repairs over the years...
I’m betting zero chance of a service center retrofiting heat pump on any model. I can’t imagine all the dependencies.